1. Field of the Invention
The present invention relates to a power supply apparatus. More particularly, the present invention relates to a power supply apparatus that controls a direct current voltage inputted from an input power supply such as a battery and has a soft start function to limit the supply power upon startup when this direct current voltage is outputted.
2. Description of Related Art
There are schemes for a power supply apparatus such as a series regulator and switching regulator. What is common to both schemes for a power supply apparatus for supplying a stable output direct current voltage to a load is a configuration detecting and feeding back the output direct current voltage, and the supply power is increased when the output direct current voltage is lower than a target value, and the supply power is decreased when the output direct voltage is higher than the target value. For this reason, upon startup when the output direct voltage is low, the power supply apparatus increases the supply power to the limit of its capacity, and therefore an inrush current flows from the input power supply. Moreover, the supply power is decreased after the output direct current voltage exceeds the target value, and therefore an overshoot is generated in the output direct current voltage due to a supply of excessive electric power.
Such a technique used to control the input inrush current and the output overshoot upon startup is the soft start function which limits the supply power upon startup.
Patent Document 1 (Japanese Patent Application Laid-Open No. 2005-269838) discloses a power supply apparatus provided with a soft start function.
FIG. 1 is a circuit diagram showing the configuration of a power supply apparatus provided with the soft start function disclosed in Patent Document 1.
In FIG. 1, the power supply apparatus has reference voltage source 1, error amplifier 2, reference signal generation circuit 3, switch circuit 4, clamping circuit 5, PWM (Pulse Width Modulation) circuit 6, input direct current voltage Vin input terminal 8, switching transistor 9, diode 10, inductor 11 and capacitor 12.
Reference voltage source 1 generates reference voltage E0 which is a target of output direct voltage Vo of the power supply apparatus.
Error amplifier 2 compares output direct current voltage Vo with reference voltage E0 and outputs error signal Ve.
Reference signal generation circuit 3 generates a reference signal that increases/decreases at a predetermined frequency.
Switch circuit 4 is turned OFF when output direct current voltage Vo reaches a predetermined value.
Clamping circuit 5 is configured with a voltage source and generates clamping voltage Vc1.
PWM circuit 6 compares error signal Ve with the reference signal and outputs a drive pulse.
Above described switching transistor 9, diode 10, inductor 11 and capacitor 12 configure a voltage conversion section referred to as a “step-down converter.” In this voltage conversion section, when switching transistor 9 repeats ON/OFF according to the drive pulse outputted from PWM circuit 6, input direct current voltage Vin is chopped, and is rectified by diode 10, and then is smoothed by inductor 11 and capacitor 12, and output direct current voltage Vo is supplied to load 13. Output direct current voltage Vo becomes higher when the rate (referred to as a “duty ratio”) of an ON time in a switching cycle of switching transistor 9 is greater.
In the above described configuration, error signal Ve outputted from error amplifier 2 that compares output direct current voltage Vo with reference voltage E0 becomes large when output direct current voltage Vo is lower than reference voltage E0, and becomes small when output direct current voltage Vo is higher than reference voltage E0. Switch circuit 4 is OFF during normal operation, and PWM circuit 6 compares error signal Ve outputted from error amplifier 2 with the reference signal outputted from reference signal generation circuit 3. By this means, the pulse width of the pulse signal outputted from PWM circuit 6 becomes wider when error signal Ve is larger.
That is, when output direct current voltage Vo is lower than reference voltage E0, error signal Ve becomes large, the duty ratio of switching transistor 9 becomes greater, and output direct current voltage Vo becomes higher. On the contrary, when output direct current voltage Vo is higher than reference voltage E0, error signal Ve decreases, the duty ratio of switching transistor 9 becomes smaller, and output direct current voltage Vo decreases. According to such a feedback operation, output direct current voltage Vo is controlled so as to be equal to reference voltage E0.
The soft start operation upon startup applies clamping voltage Vc1 of clamping circuit 5 to the input signal of PWM circuit 6 through switch circuit 4. By this means, clamping voltage Vc1 which is actually lower than high potential error signal Ve is inputted to PWM circuit 6, the duty ratio of switching transistor 9 is reduced, so that the supply power is limited. Continuing this operation until output direct current voltage Vo becomes closer to a set voltage prevents any inrush current from being generated.
However, in the power supply apparatus having such a soft start function, the supply power may be exceeded or deficient depending on load 13 and the setting of clamping voltage Vc1 inputted to PWM circuit 6.
FIG. 2 shows a timing chart of the power supply apparatus having the soft start function upon startup.
As for the power supply apparatus in FIG. 1, an overshoot is generated in output direct current voltage Vo upon startup when load 13 is a light load. Moreover, when load 13 is a heavy load, there is a problem that starting up the apparatus requires a long time.